Licence: Public Domain Mark
Credit: Qualitative chemical analysis / by C. Remigius Fresenius. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![§§ 11, 12.] the tube which is fitted into the cork; this tube is surrounded with a wider tube which is filled with cold water. In proportion as the water cools the inner tube, it becomes heated itself, and requires to be renewed either continuously or occasionally, which is done by pouring cold water in through d, after placing a vessel under g to catch the hot water which will run out, A small flask serves as a receiver. § 11. 10. Ignition. Ignition, to a certain extent, has the same relation to solids that evaporation has to liquids; since it serves, at least generally, to sepa- rate volatile from less volatile or non-volatile substances in cases where the residue alone is of importance. The process of ignition always presupposes the application of a high temperature, in which respect it differs from drying. The form or state which the eliminated substance assumes on coohng—whether it remains gaseous, as in the ignition of carbonate of lime; or assumes the liquid state, as in the ignition of hydrate of lime; or solidifies, as in the ignition of a mixture containing chloride of ammonium—is a matter of perfect indifference as regards the name given to the operation. The process of ignition is mostly employed, as has just been said, to effect the elimination of a volatile compound. In some instances, how- ever, substances are ignited simply for the purpose of modifying their state, without any volatilization taking place; thus sesquioxide of chromium is converted by ignition into the so-called insoluble modifi- cation, &c. In analytical investigations, substances under examination are often ignited, that the operator may draw some conclusion as to their nature from their behaviour at a red heat, such as their fixity, their fusibihty, the presence or absence of organic matter, &c. Crucibles are the vessels generally made use of in ignition. In operations on a large scale, Hessian or black-lead crucibles are used, heated by charcoal or gas; in analytical experiments, small-sized crucibles or dishes of porcelain, platinum, silver, or iron are chosen, or glass tubes sealed at one end, according to the nature of the substance to be ignited; these crucibles, dishes, or tubes are heated over a BerzeHus spirit-lamp, or a gas-lamp, or a bellows blowpipe. § 12. 11. Sublimation. If a solid substance is converted into vapour by the application of heat, and subsequently recondenses to a solid on cooling, the process is termed sublimation, and the substance volatilized and recondensed is called a sublimate. Sublimation may be defined, therefore, as the distillation of solid substances, and is used mostly to efiect the separation of compounds which have different degrees of volatility. It is of the gi'eatest importance in analysis for the detection of certain substances, such as arsenic. The vessels used in sublimation are of various shapes, according to the degree of volatility of the substance operated on. In sublimations for analytical purposes, glass tubes closed at one end are generally employed. When the sublimation is performed](https://iiif.wellcomecollection.org/image/b21966953_0037.jp2/full/800%2C/0/default.jpg)
